Tuesday 5 April 2011

SQL Server 2008 Interivew Questions 2

  1. Difference between char and nvarchar / char and varchar data-type?
    char[(n)] - Fixed-length non-Unicode character data with length of n bytes. n must be a value from 1 through 8,000. Storage size is n bytes. The SQL-92 synonym for char is character.
    nvarchar(n) - Variable-length Unicode character data of n characters. n must be a value from 1 through 4,000. Storage size, in bytes, is two times the number of characters entered. The data entered can be 0 characters in length. The SQL-92 synonyms for nvarchar are national char varying and national character varying.
    varchar[(n)] - Variable-length non-Unicode character data with length of n bytes. n must be a value from 1 through 8,000. Storage size is the actual length in bytes of the data entered, not n bytes. The data entered can be 0 characters in length. The SQL-92 synonyms for varchar are char varying or character varying.
  2. GUID datasize?
  3. How GUID becoming unique across machines?
    To ensure uniqueness across machines, the ID of the network card is used (among others) to compute the number.
  4. What is the difference between text and image data type?
    Text and image. Use text for character data if you need to store more than 255 characters in SQL Server 6.5, or more than 8000 in SQL Server 7.0. Use image for binary large objects (BLOBs) such as digital images. With text and image data types, the data is not stored in the row, so the limit of the page size does not apply.All that is stored in the row is a pointer to the database pages that contain the data.Individual text, ntext, and image values can be a maximum of 2-GB, which is too long to store in a single data row.

  5. What are joins?
    Sometimes we have to select data from two or more tables to make our result complete. We have to perform a join.
  6. How many types of Joins?
    Joins can be categorized as:
    • Inner joins (the typical join operation, which uses some comparison operator like = or <>). These include equi-joins and natural joins.
      Inner joins use a comparison operator to match rows from two tables based on the values in common columns from each table. For example, retrieving all rows where the student identification number is the same in both the students and courses tables.
    • Outer joins. Outer joins can be a left, a right, or full outer join.
      Outer joins are specified with one of the following sets of keywords when they are specified in the FROM clause:
      • LEFT JOIN or LEFT OUTER JOIN -The result set of a left outer join includes all the rows from the left table specified in the LEFT OUTER clause, not just the ones in which the joined columns match. When a row in the left table has no matching rows in the right table, the associated result set row contains null values for all select list columns coming from the right table.
      • RIGHT JOIN or RIGHT OUTER JOIN - A right outer join is the reverse of a left outer join. All rows from the right table are returned. Null values are returned for the left table any time a right table row has no matching row in the left table.
      • FULL JOIN or FULL OUTER JOIN - A full outer join returns all rows in both the left and right tables. Any time a row has no match in the other table, the select list columns from the other table contain null values. When there is a match between the tables, the entire result set row contains data values from the base tables.
    • Cross joins - Cross joins return all rows from the left table, each row from the left table is combined with all rows from the right table. Cross joins are also called Cartesian products. (A Cartesian join will get you a Cartesian product. A Cartesian join is when you join every row of one table to every row of another table. You can also get one by joining every row of a table to every row of itself.)
  1. What is self join?
    A table can be joined to itself in a self-join.
  2. What are the differences between UNION and JOINS?
    A join selects columns from 2 or more tables. A union selects rows.
  3. Can I improve performance by using the ANSI-style joins instead of the old-style joins?
    Code Example 1:
    select o.name, i.name
    from sysobjects o, sysindexes i
    where o.id = i.id
    Code Example 2:
    select o.name, i.name
    from sysobjects o inner join sysindexes i
    on o.id = i.id
    You will not get any performance gain by switching to the ANSI-style JOIN syntax.
    Using the ANSI-JOIN syntax gives you an important advantage: Because the join logic is cleanly separated from the filtering criteria, you can understand the query logic more quickly.
    The SQL Server old-style JOIN executes the filtering conditions before executing the joins, whereas the ANSI-style JOIN reverses this procedure (join logic precedes filtering).
    Perhaps the most compelling argument for switching to the ANSI-style JOIN is that Microsoft has explicitly stated that SQL Server will not support the old-style OUTER JOIN syntax indefinitely. Another important consideration is that the ANSI-style JOIN supports query constructions that the old-style JOIN syntax does not support.
  4. What is derived table?
    Derived tables are SELECT statements in the FROM clause referred to by an alias or a user-specified name. The result set of the SELECT in the FROM clause forms a table used by the outer SELECT statement. For example, this SELECT uses a derived table to find if any store carries all book titles in the pubs database:
    SELECT ST.stor_id, ST.stor_name
    FROM stores AS ST,
         (SELECT stor_id, COUNT(DISTINCT title_id) AS title_count
          FROM sales
          GROUP BY stor_id
         ) AS SA
    WHERE ST.stor_id = SA.stor_id
    AND SA.title_count = (SELECT COUNT(*) FROM titles)

  5. What is Stored procedure?
    A stored procedure is a set of Structured Query Language (SQL) statements that you assign a name to and store in a database in compiled form so that you can share it between a number of programs.
    • They allow modular programming.
    • They allow faster execution.
    • They can reduce network traffic.
    • They can be used as a security mechanism.
  6. What are the different types of Storage Procedure?
    1. Temporary Stored Procedures - SQL Server supports two types of temporary procedures: local and global. A local temporary procedure is visible only to the connection that created it. A global temporary procedure is available to all connections. Local temporary procedures are automatically dropped at the end of the current session. Global temporary procedures are dropped at the end of the last session using the procedure. Usually, this is when the session that created the procedure ends. Temporary procedures named with # and ## can be created by any user.
    2. System stored procedures are created and stored in the master database and have the sp_ prefix.(or xp_) System stored procedures can be executed from any database without having to qualify the stored procedure name fully using the database name master. (If any user-created stored procedure has the same name as a system stored procedure, the user-created stored procedure will never be executed.)
    3. Automatically Executing Stored Procedures - One or more stored procedures can execute automatically when SQL Server starts. The stored procedures must be created by the system administrator and executed under the sysadmin fixed server role as a background process. The procedure(s) cannot have any input parameters.
    4. User stored procedure
  1. How do I mark the stored procedure to automatic execution?
    You can use the sp_procoption system stored procedure to mark the stored procedure to automatic execution when the SQL Server will start. Only objects in the master database owned by dbo can have the startup setting changed and this option is restricted to objects that have no parameters.
    USE master
    EXEC sp_procoption 'indRebuild', 'startup', 'true')
  2. How can you optimize a stored procedure?
  3. How will know whether the SQL statements are executed?
    When used in a stored procedure, the RETURN statement can specify an integer value to return to the calling application, batch, or procedure. If no value is specified on RETURN, a stored procedure returns the value 0.  The stored procedures return a value of 0 when no errors were encountered. Any nonzero value indicates an error occurred.
  4. Why one should not prefix user stored procedures with sp_?
    It is strongly recommended that you do not create any stored procedures using sp_ as a prefix. SQL Server always looks for a stored procedure beginning with sp_ in this order:
    1. The stored procedure in the master database.
    2. The stored procedure based on any qualifiers provided (database name or owner).
    3. The stored procedure using dbo as the owner, if one is not specified.
Therefore, although the user-created stored procedure prefixed with sp_ may exist in the current database, the master database is always checked first, even if the stored procedure is qualified with the database name.
  1. What can cause a Stored procedure execution plan to become invalidated and/or fall out of cache?
    1. Server restart
    2. Plan is aged out due to low use
    3. DBCC FREEPROCCACHE (sometime desired to force it)
  2. When do one need to recompile stored procedure?
    if a new index is added from which the stored procedure might benefit, optimization does not automatically happen (until the next time the stored procedure is run after SQL Server is restarted).
  3. SQL Server provides three ways to recompile a stored procedure:
    • The sp_recompile system stored procedure forces a recompile of a stored procedure the next time it is run.
    • Creating a stored procedure that specifies the WITH RECOMPILE option in its definition indicates that SQL Server does not cache a plan for this stored procedure; the stored procedure is recompiled each time it is executed. Use the WITH RECOMPILE option when stored procedures take parameters whose values differ widely between executions of the stored procedure, resulting in different execution plans to be created each time. Use of this option is uncommon, and causes the stored procedure to execute more slowly because the stored procedure must be recompiled each time it is executed.
    • You can force the stored procedure to be recompiled by specifying the WITH RECOMPILE option when you execute the stored procedure. Use this option only if the parameter you are supplying is atypical or if the data has significantly changed since the stored procedure was created.
  1. How to find out which stored procedure is recompiling? How to stop stored procedures from recompiling?
  2. I have Two Stored Procedures SP1 and SP2 as given below. How the Transaction works, whether SP2 Transaction succeeds or fails?
    EXEC SP2

    commit tran

    Both will get roll backed.
        BEGIN TRAN
        commit tran

    Both will get roll backed.
  4. How will you handle Errors in Sql Stored Procedure?
    INSERT NonFatal VALUES (@Column2)
    IF @@ERROR <>0
      PRINT 'Error Occured'
  5. How will you raise an error in sql?
    RAISERROR - Returns a user-defined error message and sets a system flag to record that an error has occurred. Using RAISERROR, the client can either retrieve an entry from the sysmessages table or build a message dynamically with user-specified severity and state information. After the

SQL Server 2008 Interivew Questions

  1. How to know how many tables contains empno as a column in a database?
    SELECT COUNT(*) AS Counter
    FROM syscolumns
    WHERE (name = 'empno')
  2. Find duplicate rows in a table? OR I have a table with one column which has many records which are not distinct. I need to find the distinct values from that column and number of times it’s repeated.
    SELECT sid, mark, COUNT(*) AS Counter
    FROM marks
    GROUP BY sid, mark
    HAVING (COUNT(*) > 1)
  3. How to delete the rows which are duplicate (don’t delete both duplicate records).
    DELETE yourtable
    FROM yourtable a
    WHERE (SELECT COUNT(*) FROM yourtable b WHERE b.name1 = a.name1 AND b.age1 = a.age1) > 1
    WHILE @@rowcount > 0
      DELETE yourtable
      FROM yourtable a
      WHERE (SELECT COUNT(*) FROM yourtable b WHERE b.name1 = a.name1 AND b.age1 = a.age1) > 1
  4. How to find 6th highest salary
    SELECT TOP 1 salary
    FROM employee
    ORDER BY salary DESC) a
    ORDER BY salary
  5. Find top salary among two tables
    SELECT TOP 1 sal
    FROM (SELECT MAX(sal) AS sal
    FROM sal1
    SELECT MAX(sal) AS sal
    FROM sal2) a
  6. Write a query to convert all the letters in a word to upper case
    SELECT UPPER('test')
  7. Write a query to round up the values of a number. For example even if the user enters 7.1 it should be rounded up to 8.
  8. What is Index? It’s purpose?
    Indexes in databases are similar to indexes in books. In a database, an index allows the database program to find data in a table without scanning the entire table. An index in a database is a list of values in a table with the storage locations of rows in the table that contain each value. Indexes can be created on either a single column or a combination of columns in a table and are implemented in the form of B-trees. An index contains an entry with one or more columns (the search key) from each row in a table. A B-tree is sorted on the search key, and can be searched efficiently on any leading subset of the search key. For example, an index on columns A, B, C can be searched efficiently on A, on A, B, and A, B, C.
  9. Explain about Clustered and non clustered index? How to choose between a Clustered Index and a Non-Clustered Index?
    There are clustered and nonclustered indexes. A clustered index is a special type of index that reorders the way records in the table are physically stored. Therefore table can have only one clustered index. The leaf nodes of a clustered index contain the data pages.
    A nonclustered index is a special type of index in which the logical order of the index does not match the physical stored order of the rows on disk. The leaf nodes of a nonclustered index does not consist of the data pages. Instead, the leaf nodes contain index rows.
    Consider using a clustered index for:
    • Columns that contain a large number of distinct values.
    • Queries that return a range of values using operators such as BETWEEN, >, >=, <, and <=.
    • Columns that are accessed sequentially.
    • Queries that return large result sets.
      Non-clustered indexes have the same B-tree structure as clustered indexes, with two significant differences:
    • The data rows are not sorted and stored in order based on their non-clustered keys.
    • The leaf layer of a non-clustered index does not consist of the data pages. Instead, the leaf nodes contain index rows. Each index row contains the non-clustered key value and one or more row locators that point to the data row (or rows if the index is not unique) having the key value.
    • Per table only 249 non clustered indexes.
  10. Disadvantage of index?
    Every index increases the time in takes to perform INSERTS, UPDATES and DELETES, so the number of indexes should not be very much.
  11. Given a scenario that I have a 10 Clustered Index in a Table to all their 10 Columns. What are the advantages and disadvantages?
    A: Only 1 clustered index is possible.
  12. How can I enforce to use particular index?
    You can use index hint (index=<index_name>) after the table name.
    SELECT au_lname FROM authors (index=aunmind)
  13. What is Index Tuning?
    One of the hardest tasks facing database administrators is the selection of appropriate columns for non-clustered indexes. You should consider creating non-clustered indexes on any columns that are frequently referenced in the WHERE clauses of SQL statements. Other good candidates are columns referenced by JOIN and GROUP BY operations.
    You may wish to also consider creating non-clustered indexes that cover all of the columns used by certain frequently issued queries. These queries are referred to as “covered queries” and experience excellent performance gains.
    Index Tuning is the process of finding appropriate column for non-clustered indexes.
    SQL Server provides a wonderful facility known as the Index Tuning Wizard which greatly enhances the index selection process.
Difference between Index defrag and Index rebuild?
When you create an index in the database, the index information used by queries is stored in index pages. The sequential index pages are chained together by pointers from one page to the next. When changes are made to the data that affect the index, the information in the index can become scattered in the database. Rebuilding an index reorganizes the storage of the index data (and table data in the case of a clustered index) to remove fragmentation. This can improve disk performance by reducing the number of page reads required to obtain the requested data
DBCC INDEXDEFRAG - Defragments clustered and secondary indexes of the specified table or view.